There is a dog and cat repellent that works pretty well that's made by Liquid Fence. You can buy it at most garden centers or places like Wal-Mart or Home Depot. Keep in mind, though, that dogs are individuals and while some may be kept away from the product, others might find it not so effective. All you can do is try it. You could also place rocks around the plant to prevent them from digging in the area.